Classic/Antique Car Repair: 1979 MGB Brakes, wheel brakes, differential switch

Question
Braking switches unpredictably between all four wheel brakes grabbing with normal brake pedal travel, to only front wheels grabbing and very hard brake pedal actuation.  May go one way for an hour, then the other way for an hour, no obvious reason for change.  Could this be possible brake master cylinder seal problem or could it be related to the pressure differential switch?  Thanks in advance for any guidance you can offer.

Answer
When was the last time the brakes were gone through really thoroughly?

The master cylinder is immediately suspect, but if it's not being driven a whole lot (and what MGB ever is?), you're going to have to look at the calipers and wheel cylinders on the rear, too. The proportioning valve is a fairly simple device, but water or moisture in the lines can ruin any or all of these components.

Check, in this order:

Master cylinder

Calipers

Rear Wheel cylinders

Proportioning valve

Front flex lines (they often swell shut with time)

All of these components shoulds be in prime condition if you want your MGB to stop safely like a mod'n automobile!

--Paul